Tools such as an end mill, drill and boring bar are fitted to a spindle device of a machine tool through a tool holder, and are caused to move while rotating at a predetermined rotational speed relative to a work fixed on a table to thereby machine the work into a desired shape. The spindle device includes a cylindrical spindle supported in a housing so as to rotate, and a draw bar provided in the spindle and moves toward the rear of the spindle to draw the tool holder to a distal end of the spindle to clamp it. Conventional spindle devices have been disclosed in PLT 1 and 2.
A spindle device disclosed in PLT 1 is equipped with an unclamping device (draw bar drive means) which moves the draw bar toward the front of the spindle to unclamp the clamped tool holder. PLT 1 describes in paragraph 0014 that the unclamping device has a draw bar drive cylinder and a draw bar drive piston for pushing the draw bar toward the front of the spindle.
PLT 2 discloses a spindle device of a machine tool having a bearing which permits the elongation of the spindle to escape rearward. According to PLT 2, paragraph 0015, a major portion on the distal end side of the spindle is supported by a pair of angular ball bearings in a manner to rotate, and an extending portion on the rear end side of the spindle is supported by a radial ball bearing so as to manner. Referring to paragraph 0018, further, track grooves are formed in the inner circumferential surface of an outer wheel of the radial ball bearing, and the outer circumferential surface of an inner wheel is formed flat without track groove, enabling the inner wheel to move in the axial direction.